Conversation:

I get synaesthesia while in the hypnogogic/hypnopompic state - ie when surfing the cusp between the sleeping and waking state and not fully in either.

Here is where you recognise that a different mind has taken over the control room and different rules apply.

You discover that "hallucinations" happen that incorporate and blend all five senses together.

Not just the most obvious - visual - but also the less apparent ones that hardly ever feature in normal dreams, ie taste and smell. I can be lying in bed and I will smell new-mown grass, for instance, even though I'm miles away from any lawns or lawnmowing. This has, for some reason, a strong affiliation with the colour yellow and not as you might expect green.

Perhaps a synaesthete is one who carries aspects of the hupnopompic wirth them all day throughout normal waking?